1. Field of the Invention
The present invention relates to switching or DC-DC type converters. It is particularly but not exclusively applicable to the power supply for onboard systems, for example on spacecraft.
2. Description of the Prior Art
Switch-mode converters have the advantage of being compact while having a very high power conversion efficiency, which is particularly useful for onboard systems. They are frequently used in applications requiring a well regulated voltage, even when the voltage source is variable.
In some cases, the required voltage is greater than the source voltage. In this case a boost converter is used. FIG. 1 shows the block diagram for such a converter. This converter comprises a switching device M, for example an N-channel MOSFET (Metal-Oxide Semiconductor Field Effect Transistor) type device mounted in parallel, powered by a current source composed of the voltage source to be regulated connected in series to an inductor Lb, and the gate of which is connected to a control circuit 10 also connected to the transistor source, in other words the ground. The terminals of the switching device M are connected to a forward-mounted diode D mounted in series with a filter capacitor Co, the load to be powered being connected to the capacitor terminals.
The “conductance control” is a control mode currently used to control the switching device. This control mode is described particularly in the “PWM Conductance Control” document by D. O'Sullivan, H. Spruyt & A. Crausaz, IEEE PESC, 1988.
Although this boost converter is very simple and efficient, it has dynamic performance limitations due its transfer function with zeros in the complex right half plane and because it produces a pulsed output current. The result is that it is not frequently used in electrical power supplies installed on spacecraft.
In an attempt to find a solution to this problem, a filter stage was added to the output from the converter, as shown in FIG. 2. This filter stage comprises a capacitor C mounted in parallel with the diode D and the switching device M, and an inductor Lo arranged between the two capacitors C and Co. This filter stage reduces the noise present in the output current, but it does not improve the dynamic performances of the converter.
At the end of the 1980s, it was proposed to overcome this problem by integrating a capacitive energy transfer device into this converter. This type of converter, also known as two-inductor boost converter, is shown in FIG. 3. The converter shown in this Figure is different from the converter shown in FIG. 2 in that the inductor Lb is connected by being inserted between the junction point between the switching device M and the capacitor, and the ground path, rather than being connected to the converter input. In this converter, the capacitor C may be considered as a voltage source. As soon as the switching device M starts conducting, there is an immediate power transfer to the output of the circuit. This avoids effects resulting from the presence of right half plane (RHP) zeros in conventional boost converters.
In fact, the capacitor C partially discharges when the switching device is conducting, which leads to a transfer of energy to the output. The capacitor C is recharged while the switching device is open.
When the behavior of this circuit is analyzed precisely, it can be observed that its transfer function has a double pole and a double zero. Double poles and double zeros can easily be eliminated by appropriate selection of circuit components and appropriate damping, so as to obtain performances of a stable first order system. Thus, this converter is frequently used in battery discharge regulation circuits.
However, this converter has the disadvantage that it requires a floating control 10 (not connected to the ground) since the switching device M formed by a MOFSET transistor is not close to the ground path due to the presence of inductor Lb. Furthermore, since the floating terminal (connected to the inductor Lb) of the transistor alternately passes between positive and negative voltages during nominal operation of the converter, the floating control cannot be achieved simply using an existing integrated circuit since there is no integrated control circuit that resists negative voltages.